The recent trend towards smaller and higher-density electronic apparatuses is increasing the use of double-sided multi-layer boards compared with conventional one-side printed wiring boards on which electronic components are placed. Consequently, boards that can hold a higher density of circuits and components are being developed (e.g. “Marked development trend towards build-up multi-layer PWBs,” Kiyoshi Takagi, January 1997, “Surface Mounting Technology,” Nikkan Kogyo Shimbun).
Prior art is described next with reference to FIGS. 6A to 6G. Board material 61 in FIG. 6A is a prepreg in B-Stage which is made by impregnating a thermosetting material such as epoxy resin into woven glass fabric for printed circuit boards, and then drying it. Film 62 is pasted on both faces of board material 61 by lamination, typically by hot rolling.
Next, as shown in FIG. 6B, via hole 63 is created in board material 61 using processing methods such as a laser beam. Then, as shown in FIG. 6C, conductive paste 64, made typically by mixing together conductive particles such as copper powder, thermosetting resin, curing agent, and solvent, is injected into via hole 63. Film 62 is then peeled off revealing conductive paste 64 protruding as shown in FIG. 6D. Copper foil 65 is disposed on both faces over this conductive paste 64, and heated and pressed using hot pressing equipment (not illustrated). This thermally cures board material 61 and compresses conductive paste 64 such that copper foils 65 on top and rear faces are electrically connected. Here, epoxy resin impregnated in board material 61 flows outward to form leaked portion 66. An unneeded portion at the edge is then cut to make a shape as shown in FIG. 6F, and then copper foil 65 is processed into a predetermined pattern, typically by etching, to create circuit 67. The double-sided printed wiring board as shown in FIG. 6G is thus completed.
In the above manufacturing method, however, electrical connection between the top and rear faces of the printed wiring board is unsatisfactory in some cases. In addition, a similar failure occurs, in some cases, between the surface layer and inner layer circuits when a multi-layer printed wiring board is formed using the above manufacturing method.
A major cause of this failure is leaking particle 610, originally a conductive particle in conductive paste 64, flowing out of via hole 63 as shown in FIG. 6E. To realize an ideal electrical connection, conductive paste 64 needs to be compressed vertically in FIG. 6E so that conductive particles in the conductive paste make firm and effective contact, and also firmly contact copper foil 65. However, as is apparent from the formation of leaked portion 66 during the steps shown in FIGS. 6D and 6E, the thermosetting resin in board material 61 flows outward. In this state, conductive particles in conductive paste 64 are pressed and flow horizontally as in FIG. 6E, resulting in conductive paste 64 being insufficiently compressed. Accordingly, the electrical connection through conductive paste 64 is unstable. The above description refers to a board material using woven glass fabric and thermosetting resin. The same phenomenon occurs with the use of inorganic fibers other than glass fiber, organic fibers such as aramid fiber, or nonwoven fabric other than woven fabric as reinforcement.
If a woven fabric is used, the flow of thermosetting resin as described above is noticeable, since the flow resistance in woven fabric is particularly low. This makes it difficult to establish electrical connection using conductive paste. In addition, deviation of the fibers comprising woven fabric has a detrimental effect. This phenomenon is described next with reference to FIGS. 7A to 7C. As shown in FIG. 7A, via hole 63 is created by a laser beam on board material 61 containing woven glass fabric 68. Looking at this area from the top, via hole 63 is made by cutting woven glass fabric 68 as shown in FIG. 7B. The processes described using FIGS. 6C to 6E are then applied. As shown in FIG. 7C, when via hole 63 on the printed wiring board is observed after these processes, conductive paste 64 is seen to have spread around via hole 63 and woven glass fabric 68 is also moved outward from via hole 63, compared to the initial regular arrangement, due to the pressing force applied during hot pressing and the flow of impregnated resin. The occurrence of the above phenomenon impedes efficient compression of conductive paste 64. This phenomenon is thus a disadvantage, manifested as variations in electrical connection resistance and less reliability, in the manufacture of printed wiring boards.
Since thinner printed wiring boards are in increasing demand, thin woven glass fabrics are often used. Such material contains a lower density of glass fiber, which means relatively larger spaces are present between fibers, aggravating the above disadvantage. In particular, the above phenomenon becomes serious when woven glass fabrics less than 100 μm thick are used.
The major factors determining the compression rate of conductive paste 64 are the degree of compression in the thickness direction of board material 61 in the hot press processing in FIGS. 6D and 6E and the protruding distance of conductive paste 64 from board material 61 in FIG. 6D. Since there are numerous interlayer connecting points through via holes 63 on a high-density printed wiring board, another element for effecting compression of conductive paste 64 is required in addition to controlling the above two major factors.